The present invention is related to flow measurement, and in particular to ultrasonic flow measurement wherein a fluid flowing in a conduit is measured by transmitting ultrasonic waves into or across the flowing stream. Such measurement systems are widely used in process control and other situations where fluid measurement is required. In general the constraints involved in setting up any such system involve generating a well defined ultrasonic signal, coupling it into the fluid, receiving some portion of the signal after it has traveled through the fluid, and processing the detected signal to determine a parameter of interest such as flow rate, fluid density or the like. Measurement by ultrasonic signal interrogation offers several advantages, among which are the possibility of performing the measurement without installing specialized measurement cells, or even without intruding into the fluid or its container, and without causing a pressure drop or flow disturbance in the fluid line. When the situation permits the use of a transducer clamped to the outside so that no special machining is needed, then the further advantages of installation without interruption of flow, low maintenance cost and portability of the measurement system may be obtained.
However, there are many trade-offs in configuring an ultrasonic fluid measurement system. Generally, the conduit or vessel wall carries noise and may also constitute a significant short circuit signal path between transducers. Further, when the fluid has very low density, or is a gas, it carries very little signal energy compared to that in the pipe wall, and acoustic impedance mismatch may couple the signal poorly, resulting in passage of an extremely weak signal.
When the fluid to be measured is of low density, such as steam at low pressure, lower molecular weight hydrocarbon liquids, or flare gas at atmospheric pressure, the foregoing factors all apply, and the low acoustic signal across the fluid together with the high level of conduit and short circuit noise have heretofore frustrated the design of an ultrasonic flow measurement system for clamp-on application to steel pipe. While wetted transducers adjacent to the free stream may be implemented with special installation or custom spool pieces, it would appear that substantial improvements in attainable signal quality will be required before an effective external measurement system can be devised for these fluids, for flowing steam at low pressures, or for flare gas at atmospheric pressure.
It would therefore be desirable to develop an external ultrasonic system for measuring the flow of low density liquids and fluids such as steam or gas in a conduit.
It would further be desirable to develop an ultrasonic system which conveniently clamps onto a flow conduit without machining operations or interruption of the flow, and which is capable of launching and receiving signals effective to determine a flow measurement.
The present invention achieves one or more of the foregoing objects by providing first and second clamp-on ultrasonic signal transducers externally coupled or attached to a steam or gas conduit, and positioned to launch and receive contrapropagation signals along a path across the flowing fluid. The transducers apply a polarized shear wave to the conduit wall to couple energy to or from a region of the wall transmitting a strong signal into the fluid. Further, the transducers are precisely aligned with the axis along the mid plane of the conduit and when acting as a receiver each transducer has an enhanced sensitivity to coherent energy transmitted through the fluid, effectively re-polarizing energy received along the transit path. The transducers are selected to produce a well defined signal of relatively high power, and preferably in a single mode with a short pulse. The transmitted signal travels along a coupling wedge to provide a polarized shear wave that refracts into the conduit wall. The wedge is preferably a low sound speed (e.g., a polymer) wedge that couples a shear wave beam into the pipe wall at a high angle of incidence so that the vertically polarized beam produces multiple internal reflections within the wall, coherently energizing a region of the wall and radiating the transmitted signal into the fluid as a beam directed across the flow. The vertical shear (SV) configuration of the transmitter and receiver effectively discriminates to receive acoustic energy with polarization on the mid plane of the conduit. Thus, the signal crossing through the fluid maintains its polarization plane, i.e. is repolarized to the same waveform, after two mode conversions through shear-longitudinal-shear along its path from pipe to fluid to pipe. The polarization plane of a transverse wave traveling laterally around the pipe wall rotates after zigzagging inside of the curving pipe wall, and is subject to other interfering or canceling effects, so the SV assembly effectively filters out a substantial amount of the crosswalk present as horizontally polarized shear wave energy propagating along the pipe wall from the opposite transducer. This results in a substantially higher signal to noise ratio than expected, even prior to further (electrical) signal processing.
The transducer assembly may employ a dammed crystal of dimensions effective to provide an output that converts to the desired waves in the pipe wall, i.e., to a vertically polarized shear wave signal, typically at a frequency between about 50 kHz and 1 MHz, depending on the acoustic properties of the fluid and the thickness of the pipe wall. To further reduce short circuit energy inside the pipe wall, including types of plate wave and Rayleigh wave, particularly Rayleigh wave that travels effectively along the curving surface, a couplant such as a gel or gel type of high temperature damping material (polymer) is applied between the conduit and a damper (or just the couplant alone may be applied to the conduit) to couple this part of the energy from the pipe and further minimize the noise effectively. The shear wavelength in the pipe wall is advantageously less than the skip distance in the wall when the system relies on coherent reinforcement to energize a region of the wall, as described further below, to launch a strengthened signal in the fluid.
The transmitter and receiver are preferably identical assemblies, and act, together with their mounting, as polarizers so that as transmitters they effectively enhance the signal of interest and, as receivers, reject other components of the pulse burst. The transducers employ a single mode crystal or preferably a high sensitivity broad band transducer such as one formed as an array of cells constructed of a composite electroactive material, to produce, in the simplest case, a clean longitudinal pulse of relatively homogeneous power distribution across its face, and this is coupled by the wedge into an axially extending region of the conduit wall to launch the fluid-borne signal. At the receiving transducer the signal is received through a similar wedge arrangement, and the transducer output preferably also passes through a band-pass filter with center frequency at the transmission frequency.
The transducers may be coupled to the conduit via a plastic wedge to launch a mode-converted shear wave signal into the wall as a skip or zigzag signal reflecting at a steep angle, or may couple via a stainless steel wedge of appropriate geometry to determine its launch angle into the conduit. The launch angle is set so that the shear wave reflects internally in the conduit to energize a region of the pipe wall for some distance along the direction of flow, and the wedge is aligned so that the signal reflecting within the wall is coupled into the flowing fluid coherently over a region extending along the axis as the wall-borne shear wave signal reflects internally in the conduit wall. The enhanced transmission geometry allows enhanced coupling into the low density fluid, and the receiver may be positioned for signal reception at a position that is as much as one or even several pipe diameters along the flow stream to enhance resolution.
The transducer/wedge assembly is typically coupled to the conduit by a hold-down clamp, such as solid block or plate with a bottom face curved to seat on the conduit and clinched down with a strap or chain. The transducer/wedge fits in a channel of the plate, and is urged against the conduit wall by one or more set screws, locking cams or the like. In accordance with another aspect of the invention, this mounting plate is positioned over a sheet or layer of material which is both acoustically damping and thermally insulating, so that it resides at a temperature closer to ambient than that of conduit wall. The mounting plate urges the transducer into acoustic contact with the conduit wall through a window in the damping/insulating sheet or layer, while itself thermally contacting the transducer assembly and acting as a thermal sink for the transducer or wedge.